1. Field of the Invention
This invention relates to an image coding apparatus for encoding moving picture data, an image decoding apparatus for decoding moving picture data, and an image processing system for transmitting and receiving moving picture streams and regenerating the moving pictures.
2. Description of the Related Art
Transfer of data such as images and voice data through various communication media such as the Internet is becoming increasingly common. Recently, in addition to the download type transmission system that has conventionally been used for data transfer on the Internet, stream type transmission system services are becoming increasingly available.
When transmitting multimedia data, such as video files and voice files by a download type transmission system, a data file is first downloaded from a delivery server to a storage media in a receiving terminal and then played back from the storage media. A file cannot be played back until the transmission of the file is completely finished, so this system is poorly suited for extended-time playback, real-time playback, and the such.
In a stream type transmission system, while data is being transferred from the transmitting side to a receiving terminal, playback processing of the received data is executed in parallel. This system is being used for Internet services such as Internet telephone, remote TV conferencing, and video-on-demand-services.
In a stream type transmission system, for example, an MPEG stream, which has been generated through MPEG compression processing of image data, is stored in IP (Internet Protocol) packets and then they are transmitted through the Internet. The packets are received by communication terminals such as PCs, PDAs, or cellular phones, and in this relation, various applications are being developed and promoted. A technology like this is useful for video on demand, live streaming, and real-time communications like TV conferencing and TV telephoning.
The Internet transfers packets by best-effort delivery. As a result, when the amount of data to be transferred reaches the point where it becomes a burden to the network, the network may fall into an overload status, and this can result in the loss of data due to the discarding of packets. Therefore, in order to ensure network quality over the Internet, a network with a priority function capable of setting priority by packet is proposed which utilizes the router technology DiffServ (Differentiated Services) as a major element.
As an example, DiffServ is realized by the adding of a packet priority control function to a router. In the router, two queues with different priority levels are set up. Packets placed in the high-priority queue are always processed and no packet loss occurs. Packets in the low-priority queue are processed only when the high-priority queue is empty. For example, the bandwidth of a stream to be transferred by the router can be measured and this measurement is used to determine priority. For a packet exceeding 5 Mbps a mark indicating Low is attached to the ToS field or such of the IP packet header, and a mark indicating High is attached to the header of a packet of 5 Mbps or less. In the router, the packets marked with High are placed in the high-priority queue and the packets marked with Low are placed in the low-priority queue. Thus, through packet priority control, a minimum bandwidth of 5 Mbps is ensured for this stream.
In Japanese Patent Laid-Open Publication No. 2003-37837, an apparatus is disclosed, which enables simultaneous viewing of a number of images by using a low-bit-rate compatible codec method, and when a specific image is selected, displays the specific image by using a high-bit-rate compatible codec method.
The Internet treats all packets equally. Because of this, network applications, such as multi-participant-type online conversations using moving pictures experience some problems. If a network becomes congested, since all the moving picture stream packets of participants are treated equally, they are thus lost equally, and the moving picture data of the person who is speaking is also lost. For the above-mentioned DiffServ, while it is possible to set a priority for each packet, no appropriate determination standards have been set in terms of how the priority should be set for moving picture stream packets in a multi-participant-type network application like an online conversation. In addition, even when having a network with a priority control function in which the router does priority control of packets which have been given a priority, some sort of system is necessary at the terminal receiving packets so that the image of the moving picture stream having a priority status is given display priority.